Phosphorescence is a phenomenon that the light emitted by a phosphor lasts after stoppage of excitation for duration of time sufficient for light to be perceived by the eye or a detection system, i.e., 0.1 second or longer. Phosphorescence that lasts for several hours at room temperature is called long-persistent phosphorescence. A phosphor that has long-persistent phosphorescence is called a long-persistent phosphor, or a long-lasting phosphor, or a long-afterglow phosphor.
Persistent phosphorescence was discovered in the 11th century in China and Japan and in the 16th century in Europe. In persistent phosphors, two kinds of active centers are involved: emitters and traps. Emitters are centers capable of emitting radiation after being excited. Traps do not emit radiation, but store excitation energy by trapping electrons and holes and release it gradually to the emitter due to thermal stimulation. Emitters are usually a small amount of intentionally added impurity atoms or ions. Co-activators are often intentionally added to form new trapping centers to improve the persistence time and intensity of the phosphors.
The importance of persistent phosphorescence was recognized since 1960s, and various persistent phosphors in the visible spectrum have been developed since then. Known in the art of long-persistent phosphors are sulfides, aluminates, and silicates.
The first generation long-persistent phosphors, sulfides [such as ZnS:Cu (green), CaS:Bi (blue), and CaS:Eu,Tm (red)] have been practically used for several decades. The disadvantages for sulfide phosphors include short persistence duration (e.g., three hours at the longest) and instability when ultraviolet light and moisture coexist. For these reasons, the sulfides have found only limited applications such as in luminous watch and night-time display inside a house.
Recently, aluminate-based long-persistent phosphors attracted considerable attention because of their better chemical stability, higher brightness, and longer persistence time (e.g., up to 20 hours) compared to the sulfide-based phosphors. Aluminate-based long-persistent phosphors are available in green and blue regions. The popular green aluminate phosphors include SrAl2O4:Eu2+ and SrAl2O4:Eu2+,Dy3+. Known blue aluminate persistent phosphors include CaAl2O4:Eu2+,Nd3+ and SrAl4O7:Eu2+,Pr3+/Dy3+. The main drawback of these alkaline earth aluminates is that when they contact with moisture and water, hydrolysis reaction occurs quickly, which limits the out-door applications of these phosphors.
Another popular long-persistent phosphor is silicates, which are potential alternatives for the aluminates. The silicate-based phosphors include (Sr2-x,Cax)MgSi2O7:Eu2+,Dy3+ with emission tunable from cyan to blue, green, and to yellow; Ca3MgSi2O8:Eu2+,Dy3+ with afterglow band at 475 nm; MgSiO3:Eu2+,Dy3+,Mn2+ with emission at 660 nm; and Ca0.2Zn0.9Mg0.9Si2O6:Eu2+,Dy3+,Mn2+ with emission at 690 nm.
From the above list, it can be seen that all the persistent phosphors developed up to now are in the visible region. The longest wavelength is in red at 690 nm (Ca0.2Zn0.9Mg0.9Si2O6:Eu2+,Dy3+,Mn2+). Some of these visible persistent phosphors (such as SrAl2O4:Eu2+,Dy3+) have been commercialized and widely used for security signs, emergency signs, safety indication, indicators of control panels, and detection of high energy rays, and so on. In contrast, no persistent phosphors in the infrared or near infrared region are available in market.
Infrared or near infrared long-persistent phosphors have gained considerable attention in recent years because of strong military and security demands. For surveillance in night or dark environments, infrared or near infrared emitting taggants are generally used for tagging, tracking, and locating the targets of interest. For practical military and security applications, it is desirable for the taggants to possess one or more of the following characteristics. (1) The emission from the taggants should be in infrared or near infrared spectrum, which is invisible to naked eyes but is detectable to specific infrared detection devices (such as night vision goggles) from far distance. (2) The infrared or near infrared emission from the taggants should be persistent for more than 10 hours (overnight) without additional charging (excitation). Ideally, the taggants can be repeatedly charged by solar radiation in daytime. (3) The taggants should be stable enough to withstand various out-door application environments including applications in water. (4) The taggants should be able to be inserted almost anywhere, including into liquid solution, dyes, paints, inks, epoxies, and sol-gel, which can then be coated onto almost any surface for concealment. (5) The production of the phosphors should be easy and cheap. Unfortunately, up to now, no such infrared or near infrared taggants have been available.
In design of infrared or near infrared phosphors, transition metal chromium in trivalent state (i.e., Cr3+) and nickel in divalent state (Ni2+) were widely used as the luminescent centers. Chromium can emit a narrow luminescence band around 696 nm due to the transition of 2E→4A2, or a wide band in the near infrared region related to the transition of 4T2→4A2, which strongly depend on the crystalline field strength of the host. When crystal field is strong, the first excited state will be 2E term and causes luminescence properties of the materials like in Al2O3 (ruby). In weak crystal field, 4T2 term will become lowest excited state and causes broad band emission like in BeAl2O4 (alexandrite). Since the 2E→4A2 transition is a spin-forbidden transition, the lifetime is of the order of milliseconds. On the other hand, the lifetime of wide band emission, which is spin-allowed, is around microseconds. Nickel has a complicated emission spectrum due to the appearance of emission transitions from more than one level. The emission spectra of Ni2+ in the octahedral site for garnets such as Y3Al5O12 and Gd3Sc2Ga3O12 consists of three bands in near infrared due to 3T2→3A2 transition. At room temperature, the bands are broad with a maximum at 1360 nm in Y3Al5O12, 1450 nm in Gd3Sc2Ga3O12, and 1200 nm in MgAl2O4.
It has been reported that some Cr3+ doped gallates showed strong emission in the infrared. The reported gallates include La3Ga5GeO14:Cr3+, La3Ga5SiO14:Cr3+, Li(Ga,Al)5O8:Cr3+, and MgGa2O4:Cr3+. But no afterglow phenomenon was reported.